i used to have one and use it in my boxster. yes, it's safe. it's a maintainer, i.e. very low current. to charge a dead battery with that would probably take days.
just plug it into the cigarette lighter and plug the other end to an outlet.
how old is your battery? maybe time for a new one? i usually change batteries every 5 years or so. sure, they can last longer than that but i just don't want to get stuck somewhere at an inopportune moment.
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MY 2000 S, Ocean Blue, Metropol Blue, Savanah Beige.
Bought June 2000 - Sold May 2010